Steam methane reformers (SMRs) are commonly used to produce a syngas including hydrogen and carbon monoxide from a gas feedstock such as natural gas or refinery gas. The produced syngas can be further processed within the plant to yield various end products, including purified hydrogen, carbon monoxide, methanol and/or ammonia. Conventional adiabatic pre-reformers can be incorporated into SMR-based syngas plants for different reasons. For example, pre-reformers may be implemented in order to: (i) reduce the content of ethane and heavier hydrocarbons in the feedstock to the SMR, thereby reducing the propensity for carbon formation on the SMR catalyst and potentially enabling higher SMR feed temperatures or lower SMR feed steam-to-carbon molar ratios, and/or (ii) increase the production rate of syngas or syngas-derived products from the plant for a fixed SMR furnace duty.
Conventional adiabatic pre-reformers are catalytic reactors typically charged with pellet or shaped supported catalyst loaded with a high Ni content. Typical pre-reformer gas hourly space velocities (GHSVs) based on total feed at standard conditions (i.e. 60° F. and 1 atm) can range from 15,000/hr to 25,000/hr. Typical pre-reformer feeds are steam and hydrocarbon mixtures with a steam-to-carbon molar ratio of 2 to 3, preheated to between 900° and 1200° F. Temperature change across the pre-reformer is dictated by the hydrocarbons in the feed. When significant methane steam reforming occurs, temperatures decrease along the reactor due to the endothermic reaction.
In order to increase the production rate of the plant for a constant SMR furnace duty, the pre-reformer effluent must be reheated against SMR flue gas, typically to 1100°-1200° F. Installation of this reheat coil can be expensive, especially for existing plants, as the SMR flue gas duct must be opened and modified. Removing heat from the SMR flue gas through the reheat coil also reduces the amount of export steam that can be produced in the downstream flue gas boiler and sold to customers.
U.S. Pat. No. 7,037,485 B1 to Drnevich et al. relates to the use of a dual mode reactor for converting olefin containing gas for use as a feed for a steam methane reformer. The first mode is a hydrogenation mode while the second mode is a partial oxidation mode. Some pre-reforming is accomplished in the second partial oxidation mode, but the capability to substantially manipulate the syngas production rate of the plant is limited, since the steam-to-carbon molar ratios in the reactor feed are to be maintained below 0.5, due to the presence of downstream sulfur removal beds.
U.S. Pat. No. 6,335,474 B1 to Ostberg et al. relates to the use of a noble metal catalyst on a MgO and/or MgAl2O4 spinel carrier to pre-reform hydrocarbon feedstocks containing oxygen impurities.
U.S. Patent Application No. 2005/0207970 A1 to Garg et al. considers the pre-reforming of natural gas over a nickel catalyst using oxygen in an amount less than that stoichiometrically required to partially oxidize all ethane and heavier hydrocarbons in the natural gas to carbon monoxide and hydrogen. For most natural gases, this translates into oxygen-to-natural gas molar ratios of less than 0.1, an amount that would not substantially increase the production rate of the SMR-based syngas plant.